Sallen key butterworth bandpass filter

Discussion in 'Homework Help' started by nicole, Apr 29, 2013.

  1. nicole

    Thread Starter New Member

    Apr 29, 2013
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    Hi all, i have designed a butterworth bandpass filter that passes frequencies between 100Hz to 10kHz and with gain of 2 using LM741. But the bode plot of the simulation result seems incorrect. Can anyone help? Thank you.
     
    Last edited: Apr 29, 2013
  2. Jony130

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    Feb 17, 2009
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    Here you have a "body plot" that I got from LTspice.
     
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  3. nicole

    Thread Starter New Member

    Apr 29, 2013
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    Thanks for the reply. I am using NI multisim in designing my filter. Yet i unable to get the bode plot as shown in your figure. Do u know anything wrong with my circuit? I am combining a highpass and a low pass filter in designing my bandpass filter.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    The crappy and obsolete 741 opamp maxes out at about 9 kHz. It has no capability to do much of anything. Abandon it in favor of a part that was designed in the last decade instead of the last century.
     
  5. nicole

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    Apr 29, 2013
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    But the problem is the questions request me to design the filter using LM741. So can't abandon it.
     
  6. tubeguy

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    Nov 3, 2012
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    I think you need to swap R3 and R4.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Then you need to change instructors because he is an idiot for giving such an assignment. Using an LM741 you have no idea weather the response is limited by the opamp or the frequency determining components. Can you say for sure how the rolloff is affected by those things? It is extremely poor design practice to use a device so close to its limits. If you worked for me and proposed such a thing I'd have to think seriously about firing you. No joke Sherlock, I don't have time to waste on boneheaded ideas from instructors who are decades behing the times.
     
  8. The Electrician

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    Oct 9, 2007
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    Your main problem is the Bode plotter block is set for a linear horizontal axis. Set it for a logarithmic horizontal (frequency) axis.
     
  9. nicole

    Thread Starter New Member

    Apr 29, 2013
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    Ya, i did changed the axis for logarithmic horizontal but the results still not so good..
     
  10. nicole

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    Apr 29, 2013
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    ya.I swap the resistor values already. But my bode plot still incorrect.
     
  11. ScottWang

    Moderator

    Aug 23, 2012
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    Show the uA741 datasheet on page 9 to your instructor, the frequency response about 12 Khz when you supply a ±12V for it, and ask the instructor what should you do for the next step.
     
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  12. tubeguy

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    Nov 3, 2012
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    R3 and R4 still look the same as post #1. Make R3 = 10k and R4 = 20K.
     
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  13. nicole

    Thread Starter New Member

    Apr 29, 2013
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    Could you please suggest another opamp that could replace LM741 in my circuit? Thanks.
     
  14. bertus

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  15. ScottWang

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    Aug 23, 2012
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    If you wish to output the higher voltage(p-p) and over 100Khz, then you can use LF357, maybe LF356 also can reach it.

    Please see the Figure 31 on page 11 from here.
     
  16. JoeJester

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    Apr 26, 2005
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    I see that your bode plot sweeps 100 Hz to 100 kHz. The plot by jony plots from 1 Hz to 1 MHz.
     
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